So-called “direct imaging current sensors” are known for their contactless and therefore potential-free measuring of the electric currents in a conductor. These sensors detect the magnetic flux caused by the currents (for example, with a Hall sensor) in a gapped magnetic circuit and generate a signal proportional to the current strength. These sensors are very economical but have relatively low accuracy. Direct imaging current sensors are so-called “open-loop current sensors”, which do not comprise closed control circuits.
Furthermore, so-called “closed-loop current sensors” are known; in these, an opposing magnetic field with the same magnitude as the magnetic field of the current to be measured is continuously generated with the aid of a closed control circuit so that a complete magnetic field compensation is constantly brought about and the magnitude of the current to be measured can be determined from the parameters for generating the opposing field. Closed-loop current sensors therefore belong to the class of compensation current sensors.
Special types of compensation current sensors that do not contain closed control circuits are so-called “flux-gate sensors”. These current sensors comprise a magnetic core with a primary and a secondary winding. A compensation of the magnetic field, which is generated by the current to be measured that passes through the primary winding (i.e., the primary current), takes place only in certain time intervals of a measuring cycle, wherein the magnetic core is driven, in each measuring cycle, with the aid of the secondary winding into positive and negative saturation. Very precise current measurement is thus possible with the aid of such sensors because the influence of the hysteresis of the magnetic core can be eliminated by using appropriate signal processing. For this reason, flux-gate current sensors are also suitable for differential current measurement. In this case, the primary winding is composed of at least two partial windings (with opposing winding direction); the difference of the currents, which pass through the two partial windings, is measured. In the simplest case the two partial windings are straight lines (oriented antiparallel) that run through a ring core. In the case of more than two partial windings, the currents in the partial windings are subtracted or added as functions of the direction of the current flow and the orientation of the particular partial winding.
Differential current sensors can be used in residual-current devices (also referred to as “residual-current circuit breakers”). In some applications, it is necessary to check whether the differential current (residual current) to be measured has a direct current component (DC component). However, for efficient calculation of the DC component, information about the frequency of the alternating current component (AC component) is necessary. For example, to secure charging stations for electrical vehicles, residual-current sensors capable of determining DC and AC components are necessary (cf. standard IEC 62752, “In-Cable Control and Protective Device for Mode 2 Charging of Electric Road Vehicles”). There is thus a need for current sensors to be capable of determining the frequency of the AC component of the current to be measured in a simple manner.